Shape of your head: round, oval, squarish, thin, wide, etc.
Angle and length of the jawline: pointed, round, or square
Height of the forehead: distance from the eyebrows to the hairline
Distance between the eyes: generally the distance between the eyes is the same as the width of an individual eye
Distance from the eyes to the side of the head: this is also generally the same as the width of one eye
Depth of the eye sockets. You don't want your eyes to look like they are sitting on your face. It is important to get the lights and darks correct between the brow bone, the cheek bone, and the eye to convey a sense of the eyeballs seated in their sockets.
Shape and angle of the eyebrows
Length and width of the nose. Generally the length of the nose from between the brows to the bottom of the nose is the same as the length of the ears.
Distance from the bottom of the nose to the mouth: this is about one-third of the distance from the bottom of the nose to the chin.
Width and fullness of the mouth and lips
Fullness of the cheeks and angle of the cheekbones
The face is divided roughly into thirds with the hairline to the eyebrows being one-third, the eyebrows to bottom of the nose the next, and the bottom of nose to chin the next.
The outer edges of the nose line up with the inner corner of the eyes.
Generally the outer corners of the mouth line up vertically with your pupils.